Flat bed scanners are devices used for digitizing documents such that they may be viewed or edited by a computer system, for example. In a typical flat bed scanner system, the face of an original document (hereinafter referred to as the "original") is placed upon a flat, transparent reference surface. The original document is fixed on the surface such that a line of the original, herein after referred to as a "scan line," is illuminated from above. The light reflected from the scan line is directed through an optical system to form an image of the scan line on a sensor, such as a charge-coupled device (CCD) array or CMOS sensor device. The sensor converts the optical signal into an electronic representation. The electronic representation is typically a sequence of voltages that correspond to the levels of the associated pixels located along the scan line. The original is scanned by moving the illumination system, optical system and CCD sensor relative to the original, along a direction hereinafter referred to as the "scanning axis". However, systems also exist in which the original document is moved relative to a fixed optical system, i.e. sheet-feed scanners.
Recently, trilinear CCD's have become increasingly common for color scanning. The typical trilinear CCD is comprised of three rows of photosensor elements. Each row of photosensor elements, referred to as a photosensor array, is covered by a red, green or blue integral filter stripe for spectral separation. When the CCD is viewed from the end, the three color pixel sensor arrays (red, green and blue (RGB)) are separated by a physical channel or gap. That interchannel spacing is hereinafter referred to as the optical line spacing (OLS).
The three photosensor arrays are individually activated to convert the incoming light to a representative charge. After a predetermined exposure, the photodiodes that make up a photosensor array transfer their charge to the associated shift registers. That charge is then shifted to the output stage. The shift registers are associated with odd and even photodiodes and, as such, the charge contained therein is shifted to the output stage in an interlaced manner. That interlaced manner allows the charge from even and odd samples to be output in sequential order. The respective charges from each color are converted into representative analog voltages and multiplexed into a single A/D converter. Because the A/D converter can only digitize a single analog voltage, the multiplexer is incorporated into the data receiving port of that A/D converter. The control logic for that multiplexer allows the red, green, and blue voltages to sequentially pass to the actual analog to digital converter circuitry. For example, voltages representing red, green and blue sample data are placed on the three analog signal lines coupled to the multiplexer. Sequentially, each of those voltages is passed to the A/D converter circuitry and digitized. When each of the three voltages is digitized, a new set of voltages representing the next red, green and blue sample data is placed on the analog signal lines.
Because all of the analog signals pass through a single A/D converter, coupling within that A/D converter allows analog voltages from other colors to couple into the active color's analog voltage. Any noise which results from cross-coupling of any multi-color signals during image acquisition (prior to digitization) is referred to as crosstalk noise. Because each of the three analog voltages presented to the multiplexer represent pixels at different locations in the original, aberrations caused by the above mentioned sources are spread throughout the resulting digitized image. Such an aberration can sometimes be detected by the human eye. Therefore, a method and apparatus are needed for canceling crosstalk noise such that aberrations do not arise in a resulting digital image.